High voltage switch assemblies are known for use at voltages tyically in the range of from 300 KV to 1 MV, such as high pressure gas-filled spark gap switch assemblies. Whilst such known spark gap switch assemblies are capable of providing a single on/off switching action at a speed of 5 nanoseconds or less they are not capable of providing a series of successive on/off switching actions at a fast enough rate, such as up to one switching action per microsecond. Additionally such known switch assemblies also have the drawback of remaining conductive even after the switch current has decayed to zero, and can remain conductive for times in excess of 10 microseconds due to the gas remaining ionised.